Firstly, beings need the Full Coordination advantage to make more than one melee attack per round with weapons held in different hands. However, I don't see this as being a necessary advantage for firing two (or more) guns at once - any fool can do that, provided they have enough arms! The question is what penalties to assign.
Firing with off-hand = -4 penalty (page B13)Additionally, I recall a discussion on the Usenet newsgroup rec.games.frp.gurps about firing guns one-handed. I think (but may be mistaken) someone quoted a rule that says two-handed firing is the default and one-handed gunfire attracted a -2 penalty. I can't find such a rule anywhere though, but it sounds reasonable to me. If any reader can find the reference, please tell me about it!
Automatic fire one-handed = double RCL penalty (page B121)
Any weapon fired one-handed = double RCL penalty (page HT13, this should supercede the rule on page B121)
Independently Focusable Eyes advantage = may aim more than one weaon at once (page CI58)
I also can't find a rule covering the firing of more than one gun at the same time. If there is one, can some kind soul please point me to it? In lieu of an official rule, I've decided a penalty of -2 per weapon beyond the first should apply to each weapon.
Finally, I've also decided that if additional guns are fired at different targets to the first, then those weapons should receive the -5 "wild swing" penalty. The -10 "shooting blind" penalty seems a little harsh given that the character can see the targets, even though he may not be focussing attention primarily on them.
For a two-handed character this gives the following total penalties for shooting two guns at once:
Favoured hand: -4, plus double RCL (-2 for firing one-handed, -2 for the additional weapon)
Off hand: -8, plus double RCL (as above, plus -4 for off hand) OR: -13 if fired at a different target.
However, if one of a character's many favoured hands is engaged in a one-handed task, then the other favoured hands are considered off hands if they are trying to do something else. If the character has the Full Coordination advantage, then each level of Full Coordination adds one extra hand that remains favoured when applied to a different task.
For example, this leads to the following penalties for a four-armed character:
Beings with IFE can aim two (or more, depending on the level of IFE) guns, either at the same or different targets, but firing one without firing the other breaks the aim on the unfired weapons, unless the being also has Full Coordination. Firing any number of guns simultaneously allows the aiming bonus to be applied to each of those weapons for that round only. The aiming bonus is then lost for all guns other than one (character's choice), but Full Coordination will allow extra guns to retain their aim, according to the level of coordination.
Beings with IFE may choose a number of different targets up to their number of independently focusable eyes without attracting the -5 "wild shot" penalty. This is true whether the shots are aimed or not.